Support function args const generics by bit-aloo · Pull Request #162727 · rust-lang/rust

bit-aloo · 2026-09-13T14:27:49Z

This PR adds initial support for argument-position const generics behind the function_arg_const_generics feature gate.

It allows const generics to be declared directly in function arguments. The PR adds the basic plumbing across AST, HIR, and ty, parses these parameters into the function's generics, lowers call arguments into consts. For now, we lower literals, negated literals, const parameters, const items, unit variants, and etc, This works for functions, methods and a couple of more cases.

r? @BoxyUwU
cc: @khyperia

rustbot · 2026-09-24T17:29:43Z

Requested reviewer is already assigned to this pull request.

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rustbot · 2026-09-24T17:29:57Z

rustfmt is developed in its own repository. If possible, consider making this change to rust-lang/rustfmt instead.

cc @rust-lang/rustfmt

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

Changes to the size of AST and/or HIR nodes.

cc @nnethercote

HIR ty lowering was modified

cc @fmease

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

ytmimi · 2026-09-24T17:39:30Z

            ref ty,
            span,
            default,
+            arg_pos: _,


What is arg_pos, and does rustfmt need to handle formatting it in some way?

Probably good to add a #![feature(function_arg_const_generics] test case to rustfmt to make sure things are getting formatted as expected.

View changes since the review

khyperia

I'm feeling a bit overwhelmed and struggling to review this, and haven't gotten through the parser changes yet. Still though, wanted to comment for what I've seen so far. I think I'd appreciate talking over how to deal with lowering here in zulip or something, I haven't fully thought through the theory space. If you have any direction/resources/etc. on lowering (e.g. whether we fundamentally must lower from hir instead of ast, what potential issues there are from doing so), I'd love to see.

View changes since this review

rustbot · 2026-10-07T10:41:21Z

Some changes occurred in compiler/rustc_builtin_macros/src/autodiff.rs

cc @ZuseZ4

khyperia

rawr, nice! looks good to me :3 just a couple nits!

... I am realizing now that you poking in Zulip maybe didn't imply you wanted a rereview but I only realized this just now after writing all this, so uuuh, get reviewed I guess

View changes since this review

rust-timer · 2026-10-10T17:59:10Z

Finished benchmarking commit (c48133e): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	3
Regressions ❌ (secondary)	0.3%	[0.1%, 0.4%]	10
Improvements ✅ (primary)	-0.3%	[-0.6%, -0.1%]	5
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.1%]	39
All ❌✅ (primary)	-0.1%	[-0.6%, 0.1%]	8

Max RSS (memory usage)

Results (primary -1.1%, secondary -2.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	4.6%	[4.6%, 4.6%]	1
Improvements ✅ (primary)	-2.1%	[-2.7%, -1.6%]	2
Improvements ✅ (secondary)	-8.7%	[-8.7%, -8.7%]	1
All ❌✅ (primary)	-1.1%	[-2.7%, 1.1%]	3

Cycles

Results (primary 2.4%, secondary 1.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.1%, 2.6%]	2
Regressions ❌ (secondary)	3.3%	[2.1%, 4.8%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	2.4%	[2.1%, 2.6%]	2

Binary size

Results (primary -0.0%, secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	4
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	2
All ❌✅ (primary)	-0.0%	[-0.1%, -0.0%]	4

Bootstrap: 487.044s -> 489.815s (0.57%)
Artifact size: 406.45 MiB -> 406.75 MiB (0.07%)

bit-aloo · 2026-10-10T19:56:31Z

@bors try @rust-timer queue

…enerics, r=<try> Support function args const generics

rust-bors · 2026-10-10T21:00:30Z

☀️ Try build successful (CI)
Build commit: 5415718 (5415718efb0ab088d913bf332f4581c437f1cd84)
Base parent: 6e4cba3 (6e4cba3380a6f9e1e4d7672e11e40372bf04e873)

rust-timer · 2026-10-10T22:27:40Z

Finished benchmarking commit (5415718): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	0.3%	[0.1%, 0.3%]	8
Improvements ✅ (primary)	-0.3%	[-0.6%, -0.1%]	6
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.1%]	39
All ❌✅ (primary)	-0.2%	[-0.6%, 0.1%]	7

Max RSS (memory usage)

Results (primary 1.3%, secondary 2.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.9%	[0.6%, 2.8%]	8
Regressions ❌ (secondary)	3.8%	[0.9%, 9.8%]	8
Improvements ✅ (primary)	-2.9%	[-2.9%, -2.9%]	1
Improvements ✅ (secondary)	-2.8%	[-3.2%, -2.2%]	3
All ❌✅ (primary)	1.3%	[-2.9%, 2.8%]	9

Cycles

Results (secondary 4.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	7.5%	[2.8%, 11.5%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.3%	[-3.8%, -2.8%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	123
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	105
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	123

Bootstrap: 488.311s -> 489.736s (0.29%)
Artifact size: 407.22 MiB -> 406.75 MiB (-0.12%)

fmease

FnParseMode.allow_const is quite vague of a field name. If you don't look at its docs it could perceivably refer to sth. like "allow const on fn aka const fn"

Otherwise the parser changes LGTM now

View changes since this review

khyperia · 2026-10-13T20:10:18Z

@bors r+

rust-bors · 2026-10-13T20:10:21Z

📌 Commit 8104522 has been approved by khyperia

It is now in the queue for this repository.

JonathanBrouwer · 2026-10-13T21:00:16Z

@bors rollup=iffy
Making a rollup of perf sensitive PRs

…uwer Rollup of 3 perf-sensitive pull requests Successful merges: - #162727 (Support function args const generics) - #164059 (perf: drop error in `unwrap_failed` instead of `unwrap`) - #164236 (Move `fully_perform` early return earlier)

@BoxyUwU

Rollup merge of #162727 - bit-aloo:2026-09-06-function-argument-const-generics, r=khyperia Support function args const generics This PR adds initial support for argument-position const generics behind the `function_arg_const_generics` feature gate. It allows const generics to be declared directly in function arguments. The PR adds the basic plumbing across AST, HIR, and `ty`, parses these parameters into the function's generics, lowers call arguments into consts. For now, we lower literals, negated literals, const parameters, const items, unit variants, and etc, This works for functions, methods and a couple of more cases. r? @BoxyUwU cc: @khyperia

rust-timer · 2026-10-14T14:36:37Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (91a47b5): comparison URL.

Overall result: ❌ regressions - please read:

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.1%, 0.3%]	4
Regressions ❌ (secondary)	0.3%	[0.1%, 0.6%]	11
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	0.2%	[0.1%, 0.3%]	4

Max RSS (memory usage)

Results (secondary -3.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.5%	[-3.5%, -3.5%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -2.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Artifact size: 406.81 MiB -> 406.98 MiB (0.04%)

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BoxyUwU self-assigned this Sep 13, 2026

fmease reviewed Sep 13, 2026

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Comment thread compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs Outdated

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rust-cloud-vms Bot force-pushed the 2026-09-06-function-argument-const-generics branch from 069cb06 to 0816e30 Compare September 14, 2026 10:55

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bit-aloo mentioned this pull request Sep 24, 2026

Tracking Issue for function_arg_const_generics #163261

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bit-aloo marked this pull request as ready for review September 24, 2026 17:29

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BoxyUwU assigned khyperia Sep 28, 2026

khyperia self-requested a review September 28, 2026 15:11

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Full Const Generics rust-lang/goals#100

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Comment thread compiler/rustc_ast/src/ast.rs Outdated

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Comment thread compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

rust-cloud-vms Bot force-pushed the 2026-09-06-function-argument-const-generics branch from 74ba4f6 to e0fc639 Compare October 7, 2026 10:41

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rust-cloud-vms Bot force-pushed the 2026-09-06-function-argument-const-generics branch 2 times, most recently from 9d3480a to 08a6a30 Compare October 9, 2026 09:08

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 10, 2026

use is-keyword instead of check keyword

66b0ea0

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rust-bors Bot pushed a commit that referenced this pull request Oct 10, 2026

Auto merge of #162727 - bit-aloo:2026-09-06-function-argument-const-g…

5415718

…enerics, r=<try> Support function args const generics

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 10, 2026

bit-aloo requested a review from fmease October 12, 2026 20:44

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bit-aloo added 2 commits October 13, 2026 11:41

Rename allow_const to allow_const_params in FnParseMode

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Add a page about function-arg-const-generics in unstable book

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rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 13, 2026

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Rollup of 3 perf-sensitive pull requests #164285

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rust-bors Bot merged commit 49c9894 into rust-lang:main Oct 14, 2026
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bit-aloo deleted the 2026-09-06-function-argument-const-generics branch October 14, 2026 11:49

fmease mentioned this pull request Oct 14, 2026

ICE on const fn param of type ... (C-variadics) #164319

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rust-timer commented Oct 10, 2026

Overall result: ❌✅ regressions and improvements - please read:

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Overall result: ❌✅ regressions and improvements - please read:

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Overall result: ❌ regressions - please read:

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